/*
 * Yama Linux Security Module
 *
 * Author: Kees Cook <keescook@chromium.org>
 *
 * Copyright (C) 2010 Canonical, Ltd.
 * Copyright (C) 2011 The Chromium OS Authors.
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License version 2, as
 * published by the Free Software Foundation.
 *
 */

#include <linux/security.h>
#include <linux/sysctl.h>
#include <linux/ptrace.h>
#include <linux/prctl.h>
#include <linux/ratelimit.h>

#define YAMA_SCOPE_DISABLED     0
#define YAMA_SCOPE_RELATIONAL   1
#define YAMA_SCOPE_CAPABILITY   2
#define YAMA_SCOPE_NO_ATTACH    3

static int ptrace_scope = YAMA_SCOPE_RELATIONAL;

/* describe a ptrace relationship for potential exception */
struct ptrace_relation {
        struct task_struct *tracer;
        struct task_struct *tracee;
        struct list_head node;
};

static LIST_HEAD(ptracer_relations);
static DEFINE_SPINLOCK(ptracer_relations_lock);

/**
 * yama_ptracer_add - add/replace an exception for this tracer/tracee pair
 * @tracer: the task_struct of the process doing the ptrace
 * @tracee: the task_struct of the process to be ptraced
 *
 * Each tracee can have, at most, one tracer registered. Each time this
 * is called, the prior registered tracer will be replaced for the tracee.
 *
 * Returns 0 if relationship was added, -ve on error.
 */
static int yama_ptracer_add(struct task_struct *tracer,
                            struct task_struct *tracee)
{
        int rc = 0;
        struct ptrace_relation *added;
        struct ptrace_relation *entry, *relation = NULL;

        added = kmalloc(sizeof(*added), GFP_KERNEL);
        if (!added)
                return -ENOMEM;

        spin_lock_bh(&ptracer_relations_lock);
        list_for_each_entry(entry, &ptracer_relations, node)
                if (entry->tracee == tracee) {
                        relation = entry;
                        break;
                }
        if (!relation) {
                relation = added;
                relation->tracee = tracee;
                list_add(&relation->node, &ptracer_relations);
        }
        relation->tracer = tracer;

        spin_unlock_bh(&ptracer_relations_lock);
        if (added != relation)
                kfree(added);

        return rc;
}

/**
 * yama_ptracer_del - remove exceptions related to the given tasks
 * @tracer: remove any relation where tracer task matches
 * @tracee: remove any relation where tracee task matches
 */
static void yama_ptracer_del(struct task_struct *tracer,
                             struct task_struct *tracee)
{
        struct ptrace_relation *relation, *safe;

        spin_lock_bh(&ptracer_relations_lock);
        list_for_each_entry_safe(relation, safe, &ptracer_relations, node)
                if (relation->tracee == tracee ||
                    (tracer && relation->tracer == tracer)) {
                        list_del(&relation->node);
                        kfree(relation);
                }
        spin_unlock_bh(&ptracer_relations_lock);
}

/**
 * yama_task_free - check for task_pid to remove from exception list
 * @task: task being removed
 */
static void yama_task_free(struct task_struct *task)
{
        yama_ptracer_del(task, task);
}

/**
 * yama_task_prctl - check for Yama-specific prctl operations
 * @option: operation
 * @arg2: argument
 * @arg3: argument
 * @arg4: argument
 * @arg5: argument
 *
 * Return 0 on success, -ve on error.  -ENOSYS is returned when Yama
 * does not handle the given option.
 */
static int yama_task_prctl(int option, unsigned long arg2, unsigned long arg3,
                           unsigned long arg4, unsigned long arg5)
{
        int rc;
        struct task_struct *myself = current;

        rc = cap_task_prctl(option, arg2, arg3, arg4, arg5);
        if (rc != -ENOSYS)
                return rc;

        switch (option) {
        case PR_SET_PTRACER:
                /* Since a thread can call prctl(), find the group leader
                 * before calling _add() or _del() on it, since we want
                 * process-level granularity of control. The tracer group
                 * leader checking is handled later when walking the ancestry
                 * at the time of PTRACE_ATTACH check.
                 */
                rcu_read_lock();
                if (!thread_group_leader(myself))
                        myself = rcu_dereference(myself->group_leader);
                get_task_struct(myself);
                rcu_read_unlock();

                if (arg2 == 0) {
                        yama_ptracer_del(NULL, myself);
                        rc = 0;
                } else if (arg2 == PR_SET_PTRACER_ANY || (int)arg2 == -1) {
                        rc = yama_ptracer_add(NULL, myself);
                } else {
                        struct task_struct *tracer;

                        rcu_read_lock();
                        tracer = find_task_by_vpid(arg2);
                        if (tracer)
                                get_task_struct(tracer);
                        else
                                rc = -EINVAL;
                        rcu_read_unlock();

                        if (tracer) {
                                rc = yama_ptracer_add(tracer, myself);
                                put_task_struct(tracer);
                        }
                }

                put_task_struct(myself);
                break;
        }

        return rc;
}

/**
 * task_is_descendant - walk up a process family tree looking for a match
 * @parent: the process to compare against while walking up from child
 * @child: the process to start from while looking upwards for parent
 *
 * Returns 1 if child is a descendant of parent, 0 if not.
 */
static int task_is_descendant(struct task_struct *parent,
                              struct task_struct *child)
{
        int rc = 0;
        struct task_struct *walker = child;

        if (!parent || !child)
                return 0;

        rcu_read_lock();
        if (!thread_group_leader(parent))
                parent = rcu_dereference(parent->group_leader);
        while (walker->pid > 0) {
                if (!thread_group_leader(walker))
                        walker = rcu_dereference(walker->group_leader);
                if (walker == parent) {
                        rc = 1;
                        break;
                }
                walker = rcu_dereference(walker->real_parent);
        }
        rcu_read_unlock();

        return rc;
}

/**
 * ptracer_exception_found - tracer registered as exception for this tracee
 * @tracer: the task_struct of the process attempting ptrace
 * @tracee: the task_struct of the process to be ptraced
 *
 * Returns 1 if tracer has is ptracer exception ancestor for tracee.
 */
static int ptracer_exception_found(struct task_struct *tracer,
                                   struct task_struct *tracee)
{
        int rc = 0;
        struct ptrace_relation *relation;
        struct task_struct *parent = NULL;
        bool found = false;

        spin_lock_bh(&ptracer_relations_lock);
        rcu_read_lock();
        if (!thread_group_leader(tracee))
                tracee = rcu_dereference(tracee->group_leader);
        list_for_each_entry(relation, &ptracer_relations, node)
                if (relation->tracee == tracee) {
                        parent = relation->tracer;
                        found = true;
                        break;
                }

        if (found && (parent == NULL || task_is_descendant(parent, tracer)))
                rc = 1;
        rcu_read_unlock();
        spin_unlock_bh(&ptracer_relations_lock);

        return rc;
}

/**
 * yama_ptrace_access_check - validate PTRACE_ATTACH calls
 * @child: task that current task is attempting to ptrace
 * @mode: ptrace attach mode
 *
 * Returns 0 if following the ptrace is allowed, -ve on error.
 */
static int yama_ptrace_access_check(struct task_struct *child,
                                    unsigned int mode)
{
        int rc;

        /* If standard caps disallows it, so does Yama.  We should
         * only tighten restrictions further.
         */
        rc = cap_ptrace_access_check(child, mode);
        if (rc)
                return rc;

        /* require ptrace target be a child of ptracer on attach */
        if (mode == PTRACE_MODE_ATTACH) {
                switch (ptrace_scope) {
                case YAMA_SCOPE_DISABLED:
                        /* No additional restrictions. */
                        break;
                case YAMA_SCOPE_RELATIONAL:
                        if (!task_is_descendant(current, child) &&
                            !ptracer_exception_found(current, child) &&
                            !ns_capable(task_user_ns(child), CAP_SYS_PTRACE))
                                rc = -EPERM;
                        break;
                case YAMA_SCOPE_CAPABILITY:
                        if (!ns_capable(task_user_ns(child), CAP_SYS_PTRACE))
                                rc = -EPERM;
                        break;
                case YAMA_SCOPE_NO_ATTACH:
                default:
                        rc = -EPERM;
                        break;
                }
        }

        if (rc) {
                printk_ratelimited(KERN_NOTICE
                        "ptrace of pid %d was attempted by: %s (pid %d)\n",
                        child->pid, current->comm, current->pid);
        }

        return rc;
}

/**
 * yama_ptrace_traceme - validate PTRACE_TRACEME calls
 * @parent: task that will become the ptracer of the current task
 *
 * Returns 0 if following the ptrace is allowed, -ve on error.
 */
static int yama_ptrace_traceme(struct task_struct *parent)
{
        int rc;

        /* If standard caps disallows it, so does Yama.  We should
         * only tighten restrictions further.
         */
        rc = cap_ptrace_traceme(parent);
        if (rc)
                return rc;

        /* Only disallow PTRACE_TRACEME on more aggressive settings. */
        switch (ptrace_scope) {
        case YAMA_SCOPE_CAPABILITY:
                if (!ns_capable(task_user_ns(parent), CAP_SYS_PTRACE))
                        rc = -EPERM;
                break;
        case YAMA_SCOPE_NO_ATTACH:
                rc = -EPERM;
                break;
        }

        if (rc) {
                printk_ratelimited(KERN_NOTICE
                        "ptraceme of pid %d was attempted by: %s (pid %d)\n",
                        current->pid, parent->comm, parent->pid);
        }

        return rc;
}

static struct security_operations yama_ops = {
        .name =                 "yama",

        .ptrace_access_check =  yama_ptrace_access_check,
        .ptrace_traceme =       yama_ptrace_traceme,
        .task_prctl =           yama_task_prctl,
        .task_free =            yama_task_free,
};

#ifdef CONFIG_SYSCTL
static int yama_dointvec_minmax(struct ctl_table *table, int write,
                                void __user *buffer, size_t *lenp, loff_t *ppos)
{
        int rc;

        if (write && !capable(CAP_SYS_PTRACE))
                return -EPERM;

        rc = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
        if (rc)
                return rc;

        /* Lock the max value if it ever gets set. */
        if (write && *(int *)table->data == *(int *)table->extra2)
                table->extra1 = table->extra2;

        return rc;
}

static int zero;
static int max_scope = YAMA_SCOPE_NO_ATTACH;

struct ctl_path yama_sysctl_path[] = {
        { .procname = "kernel", },
        { .procname = "yama", },
        { }
};

static struct ctl_table yama_sysctl_table[] = {
        {
                .procname       = "ptrace_scope",
                .data           = &ptrace_scope,
                .maxlen         = sizeof(int),
                .mode           = 0644,
                .proc_handler   = yama_dointvec_minmax,
                .extra1         = &zero,
                .extra2         = &max_scope,
        },
        { }
};
#endif /* CONFIG_SYSCTL */

static __init int yama_init(void)
{
        if (!security_module_enable(&yama_ops))
                return 0;

        printk(KERN_INFO "Yama: becoming mindful.\n");

        if (register_security(&yama_ops))
                panic("Yama: kernel registration failed.\n");

#ifdef CONFIG_SYSCTL
        if (!register_sysctl_paths(yama_sysctl_path, yama_sysctl_table))
                panic("Yama: sysctl registration failed.\n");
#endif

        return 0;
}

security_initcall(yama_init);